Plating zinc-tin alloy



Patented Sept. 6, 1932 KUNITLED STATESIPATEN'II'; orrica;

MERRILL M. THOMPSON AND JAMES C. .PrXTTEN, F INDIANAPOLIS, INDIANA; SAID THOMPSON ASSIGNOR TO SAID PATTEN; MARGARET LUGILE' PATTEN EXEGUTRIX OF SAID JAMES C. PATTEN, DECEASED '.PLATING ZINC- TIN ALLOY No Drawing.

This invention relates to plating zinc-tin alloys;. and it comprises an improvement in methods of electrically depositing zinc-tin alloy coatings upon iron, steel and other 5. metals wherein soluble anodes of zinc-tin alloys are used and these anodes are amalgamated with mercury, the plating solution being usually -alkaline and usually containing a substantial amount of sulfonated castoroil (Turkey red oil) all as more fully hereinafter set forth and as claimed. The protection against corrosion afforded to iron and steel by an electroplated coating depends partly upon its continuity and partly upon the position of the coating metal in the electrochemical scale. Copper coatings, for example, owe their entire value to their continuity, since copper is electronegative to iron and, witha break orpore in the coating, corrosion is accelerated. Zinc, being highly electropositive to iron, protects not only because of'lts shielding value but because corrosion is restricted to zinc Where both zinc and iron are exposed. .However,-this also means that presence of iron; any kind of pore or break in the surface tending to enlarge. Cadmiumwhich is probably due, in part, to the position of cadmium relative to iron in the-electrochemicalscale. It does not tend to accelerate corrosion of the iron. Cadmium coatings have a much greater protective value than zinc coatings. Standard salt spray tests on electrodeposited zinc coatings on iron or steel sheets indicate an average life of about 200 hours with a coating of about 0.0012 inches, while much thinner cadmium coatings, say, only 0.0002 inches will give a life of 500 to 1,000 hours. A cadmium coating on iron of 0.0003 inches is sometimes regarded as equivalent, for protective purposes, to a zinc coating of 0.001 inches.

Cadmium is, however, a relatively, expen- Z sive metal; much more expensive than z1nc. It is a desirable object in the art to provide a coating better than one of zinc and less expenthe zinc corrodes more quickly because of the Application filed September 2, 1930. Serial No. 479,414.

sive-than oneof cadmium. In another and copending application Ser. No.433,500, on

which the present invention is in some aspects an improvement, this object is obtained by provldlng iron and steel with an electrically 56 deposited coating of an alloy of zinc and tin.

The ratio of zinc to tin may vary between 93:7 and :35. The particular ratio depends upon the properties desired in the coating. Deposition is from [an equi-potential '60 bath of alkaline character containing a pyrophosphate and a cyanid. .An equi-potential bath is one in which electrically connected bars of'tin and zinc show development of no substantial potential. Replenishment of the 65 batlfmay be by alloy anodes or by pairs of zinc-and tin anodes having the desired ratio of surface.

The appearance of articles electrically coated with zinc-tin alloys is extremely decorative; They can be readily buffed and polished. The protection given the underlying metal in case of iron and steel is of the same order as that afforded by cadmium.

.The coating is advantageous on other metals 75,

than iron and steel, as upon copper and brass. The coating is an excellent base for deposition of chromium or of nickel, serving for this purpose like the cadmium of Patent N o. 1,615,585. so In electrical deposition of these alloys, it hasbeen found useful to amalgamate the soluble anodes used, thus promoting the desired proportioned solution of the two metals, zinc and tin. Amalgamation also reduces nonelectric attack of the bath on the anode. As these baths are customarily quite alkaline, this is desirable. Both zinc and tin being soluble in mercury, it is probable that in the use of the amalgamated alloy anodes the zinc and tin passing into solution in the bath as an intermediate phase go into solution in the mercury; that the active anode is really a mercury solution of the'two metals.

With the use of amalgamated alloy anodes, it is found that the equi-potential character of the bath is of less importance and that ordinary salines may be used; ordinary salts such dissolved in a solution of caustic soda.

,With any alkaline bath for the deposition of zinc-tin alloys, it is found that the presence of an addition agent is of advantage.

' The best of these addition agents, so far as is at present known, is sulfonated or sulfated castoroil or ordinary commercial Turkeyred oil. In making Turkey red oil, other oils than castor oil may beemployed. Sulfonated mineral oils and sulfonated aromatic compounds may be used for the present purposes. Where the sulfonated preparations are acid, they react with the bath and it is necessary to use enough alkali toredissolve any precipitate which they may form.

While, as stated, zinc-tin alloys with widely varying ratios may be deposited and are desirable for various purposes, the ratio vary-v ing'from perhaps 93 7 to 65: 35, it has been found that in using the present invention a ratio of 90:10 or thereabouts, is most desirsired, 810 grams of commercial crystallized chlorid (SnGl QH O) zinc sulfate (ZnSOJI-I O) are dissolved in water and 35 grams of commercial stannous are dissolved in water. The solutions are mixed and to the mixture is added a solution of 1500 grams of commercial sodium hydroxide. Enough water is then added to make the volume 10 liters. A white precipitate is formed at first on the addition of the alkali solution, but this quickly redissolves. To the 10 liter bath thus formed is next added 100 cc. of commercial liquid Turkey red oil. From this'bath a coating of alloy in the 90: 10 ratio may be deposited and by the use of alloy anodes having the same ratio, the character of the bath remains unchanged over long periods of time;

The anodes used in the present invention are prepared by melting together the desired amounts of zinc and tin in the ratio of 90: 10. The molten alloy is cast into a suitable mold. For amalgamation the cast anodes thus produced are immersed in a slightly acid solution of mercurous or mercuric nitrate. The amount of dissolved mercury should be equal to about 2 per cent of the weight of the anodes. The anodes are left in the mercury solution for several hours or until the solution is stripped of mercury. The anodes may be left in the solution over night. Amalgamated anodes made as described give good results in any sufficiently alkaline bath, dissolving only when the current is on and giving up to the bath zinc and tin in the ratio in which they are contained in the alloy. They afford good coatings. The Turkey red oil in the solution is an eflicient addition agent and much promotes deposition of compact reguline metal. The cathode efliciency varies between 90 and 95 per cent. For ordinary thicknesses of coating using a current density of 10 to 20 amperes per square foot at 6 volts, about 30 minutes plating time will give a coating 0.0002 to 0.0004. inches thick which is sufiicient for all ordinary purposes. On such a coating, after buffing and polishing, an impervious chromium or nickel coating may be deposited by the ordinary coating methods.

The throwing power of a bath'under the present invention is unusually good. The coatings formed are uniform, flawless and poreless. As regards protecting the underlying metal they are better than those by the electrolytlc deposition of zinc from the stand-,

ard baths; and stand up better. For example where a .zinc coating of 0.0003 inches on Armco iron will stand up under a salt spray for about 100 hours, a similar thickness of coating of zinc-tin alloy will stand up about 1000 hours under the same conditions.

What we claim is:

1. In the electroplating of iron and steel articles with a protective coating of alloy,

the process which comprises passing current between anodes and the said articles serving as cathodes, said cathodes and anodes being immersed in a bath containing zinc and tin in solution and said anodes being a zinc-tin .alloy amalgamated with mercury.

2. In the electroplating of iron and steel articles with a zinc-tin alloy the process which comprises passing current through a bath containing zinc and tin in solution and between a plurality of anodes and the said articles serving as cathodes, the said anodes being a zinc-tin alloy amalgamated with mercury.

3. In the electroplating of iron and steel articles with a zinc-tin alloy the process which comprises passing an electric current through a solution comprisin zinc andtin in solution and a sulfonated 011, between anodes and said articles serving as cathodes, the said anodes comprising a zinc-tin alloy amalgamated with mercury.

4. In the electroplating of iron and steel articles with a zinc-tin alloy the process which comprises passing an electric current through an alkaline solution comprising zinc sulfate and stannous chlorid in solution and a sulfonated castor oil, between anodes and the said articles serving as cathodes, the said anodes comprising a zinc-tin alloy amalgamated with mercury.

5. In the electroplating of ir n and steel articles with zinc-tin alloys at predetermined metallic ratios, the process which comprises passing current through a bath between anodes and the said articles serving as cathodes, the said bath containing zinc and in approximately said predetermined ratio,

and the said anode being a zinc-tin alloy amalgamated with mercury, the zinc and tin of said alloy being in a ratio of 90:10.

6. In the electroplating of iron and steel articles with zinc-tin alloys at predetermined metallic ratios, the process which comprises passing current through a bath between anodes and the said articles serving as cathodes, the said bath containing zinc and tin in approximately said predetermined ratio, and

the said anode being a zinc-tin alloy amalgamated with mercury, the zinc and tin of said alloy being in a ratio of 10 and the amount of mercury being about 2 per cent of the alloy.

7. As a new anode suitable for electroplating iron and steel articles with zinc-tin al loys, an anode comprising a zinc-tin alloy amalgamated with mercury.

8. As a new anode suitable for electroplating iron and steel articles with zinc-tin alloys, an anode comprising a zinc-tin alloy amalgamated with mercury, the zinc and tin of said alloy being in the ratio of 90: 10.

9. As a new anode suitable for electroplating iron and steel articles with zinc-tin alloys, an anode comprising a zinc-tin alloy amalgamated with mercury, the zinc and tin of said alloy being in the ratio of 90: 10 and the amount of mercury being about 2 per cent of the alloy.

In testimony whereof, we have hereunto aifixed out signatures.

MERRILL -M. THOMPSON. JAMES C. PATTEN. 

